The mechanisms responsible for the regulation of the synthesis, assembly, and secretion of mouse immunoglobulins will be investigated by characterizing the defects in a series of mutants blocked in one or another of these processes. The mutants have already been isolated from untreated and mutagenized clones of the MPC-11 and other mouse myeloma cell lines. They are being genetically analyzed by determining their stability and ability to complement each other by fusion. Their biochemical defects will now be studied using the techniques of cell and molecular biology. Even if the genetic basis for the variants cannot be established, they provide an ideal tool for such biochemical studies.